Industrial systems as well as automotive and truck applications require monitoring of various gas compositions, such as O2, NOx, CO and CO2, in a combustion environment to insure efficient, non-polluting operations. For instance, carefully controlling the oxygen level in boilers can maximize energy output and minimize pollutant emissions. It is estimated that yearly savings of $409 million from coal-fired power plants could be enabled through combustion optimization. In view of increasing commercial demands and legal requirements for decrease of pollutants, such sensor systems are extremely important. For example NOx(NO+NO2) sensing is considered as one of the key elements of next generation internal combustion engines; and thus accurate and reliable NOx sensors must be developed to monitor NOx breakthrough to activate regeneration of NOx absorption catalysts and/or to control the injection of reductants for continuous NOx reduction. Further, for automotive and truck applications, the monitoring and control of both NOx and O2 is necessary for emission control and/or air-to-fuel ratio measurement to maximize efficiency and reduce pollutants.
Conventional oxygen sensors require a continuous source of reference air to measure accurately the content of oxygen in the combustion of gases. This requirement increases the complexity and cost of oxygen sensing systems. Most high temperature sensors capable of detecting O2 and NOx are based on stabilized zirconia, which has high ionic conductivity, as well as good mechanical and chemical stability at high temperatures. Zirconia oxygen sensors have long been used to monitor the performance of internal combustion engines in automobiles in order to increase the fuel efficiency and minimize emissions. The use of an external oxygen reference is the most common approach to detect oxygen. In this design, air is provided from outside the combustion environment to a reference electrode that is separated from the sensing environment by a zirconia channel. The oxygen concentration differential between the outside and the measuring environment generates an open circuit potential that obeys the Nernst law, allowing for direct calculation of the unknown concentration of oxygen. For NOx detection, a multistage configuration in which gases in the combustion environment diffuse through a narrow channel into one or two chambers constructed of laminated YSZ (yttria-stabilized zirconia) sheets. The first chamber is normally equipped with oxygen pumping electrodes, which can selectively remove oxygen from the gas mixture to minimize the oxygen interference. A pair of noble metal electrodes then electrochemically converts the NOx mixture into NO or NO2 exclusively, which is detected by either potentiometric or amperometric methods at the last stage.
Nevertheless, although great effort has been devoted to develop NOx/O2 dual sensors with the multi-stage configuration, a commercially successful product has still not been achieved. In order to construct gas chambers and introduce reference air from the external environment, a number of YSZ sheets and insulation layers (up to about thirteen separate layers) need to be well aligned and laminated. This process drastically increases the complexity of sensor fabrication and compromises the sensor durability under thermal shock and thermal cycles. On the other hand, it is beneficial to use more than one gas sensor in more than one location to optimize performance in multi-cylinder engines. A sensor that requires external air constrains the location of sensors inside the combustion environment and impedes the development toward sensor miniaturization. Consequently, a system meeting all the increasing needs of industrial applications has yet to be achieved.